1. Field of the Invention
The present invention relates to an SDH (Synchronous Digital Hierarchy) radio communication a system and transmitter/receiver equipment therefor.
2. Description of the Prior Art
With recent expansion of worldwide digital communication networks, CCITT (International Telegraph and Telephone Consultative Committee), recommends an SDH rule to hierarchically multiplex channels upon signal multiplexing in digital communication for interconnection among digital communications. The SDH is to multiplex modules, each called an STM (Synchronous Transport module), for digital communication. CCITT recommends the use of three kinds of STMs: STM-1 (Synchronous Transport Module Level 1 with a bit rate of 155520 kb/s), STM-4 (Synchronous Transport Module level 4 with a bit rate of 622080 kb/s), and STM-16 (Synchronous Transport Module level 16 with a bit rate of 2488320 kb/s). In cooperation therewith, TTC (Telegraph Telephone Committee) in Japan specifies STM-0 (Synchronous Transport Module level 0 with a bit rate of 51840 kb/s) as a lower level of STM-1. Hereinafter, STM-0 , STM-4 or STM-16 is expressed as STM-N and a frame of STM-N is expressed as STM frame.
The STM includes information called SOH (Section Overhead) in addition to data to be communicated. The SOH is comprised of an STM frame synchronizing byte for synchronization between STM frames on the sides of transmission and reception, and of a OW (Order Wire) for delivery of information among nodes.
It is herein noted that prior art radio communication with use of SDH is adapted such that an STM frame is divided into subframes of a predetermined size, to each of which subframes there is added an RFCOH(Radio Frame Complementary Overhead) composed of a radio frame synchronizing bits for synchronization of a radio frame on the sides of transmission and reception and an error control bits for correcting transmission error, and each of which subframes serves as a radio frame for communication.
The prior art communication system includes as described above synchronizing bytes (STM frame synchronizing byte and radio frame synchronizing byte) each for synchronization of the STM frame and of the sizes radio frame, those bytes having no relation to the of the STM frame and the radio frame.
Referring to FIG. 1, there is illusrated a block diagram of a transmitter/receiver apparatus for use in a prior art SDH radio communication system.
A transmitting station serves as follows.
An STM frame synchronization circuit 2A detects STM frame synchronizing byte in an SOH of an STM frame 101A as inputted and issues an STM frame pulse 102A at a timing when the afore-mentioned STM frame synchronizing byte is detected. An STM signal processing circuit 1A extracts the SOH from the STM frame 101A at a timing of the STM frame pulse 102A, and again inserts the SOH into the STM frame 101A after the SOH is subject to a required processing by another circuit not illustrated and issues it as an STM frame 101B.
A radio frame pulse generating circuit 4A frequency-divides a rate converting clock 104A supplied from a rate converting circuit 3A, and issues a radio frame pulse 106A. The rate converting circuit 3A forms in the STM frame 101B empty time slots for the RFCOH on the basis of the radio frame pulse 106A together with rate conversion of the same STM frame 101B, and inserts a predetermined RFCOH into the empty time slots so formed and issues it as a radio frame 107.
A receiving station serves as follows.
A radio frame synchronization circuit 5B detects the radio frame synchronizing bits in the RFCOH of the received radio frame 107, and issues a radio frame pulse 106B at a timing when the radio frame synchronizing bits are detected. A rate converting circuit 3B extracts the RFCOH from the radio frame 107 at a timing of the radio frame pulse 106B, together with the elimination of the time slot for the RFCOH and the rate conversion of the same to issue it as an STM frame 101C.
An STM frame synchronous circuit 2B detects the STM frame synchronizing byte in the SOH of the STM frame 101C, and issues an STM frame pulse 102B at a timing when the STM frame synchronizing byte is detected. A STM signal processing circuit 1B extracts the SOH from the STM frame 101C at the timing of the STM frame pulse 102B, and again inserts the SOH into the STM frame 101C after processing as required the SOH with use of another circuit (not shown) and issues the same as an STM frame 101D.
Additionally, in the prior art system, in the case where the SOH of the STM frame is terminated in an intermediate radio station installed between the transmitting and receiving stations (for example in the case where information set in the OW is extracted), it was done after rate conversion.
In the afore-mentioned prior art SDH radio communication system, it involves bytes in the STM frame and the radio frame for synchronization therebetween (the STM frame synchronizing byte in the SOH and the radio frame synchronizing byte in the RFCOH), and has no relation to the sizes of the STM frame and the radio frame. Accordingly, in the case where the SOH of the STM frame is terminated in the intermediate radio station installed between the transmitting and receiving stations, it requires rate conversion. More specifically, the intermediate station requires a rate converting circuit to result in complicated circuit construction followed by serverely greater power consumption.
Furthermore, in a transmitter/receiver equipment for use in the prior art SDH radio communication system, it adopts double frame synchronizations of those for the STM frame and the radio frame, so that much time is required for the synchronization together with the need of two frame synchronous circuits 2A and 2B to result in complicated circuit construction and severely greater power consumption.